Traditional magnetic recording systems use a preamble pattern for read channel synchronization, in both servo and data fields. In such traditional systems, this preamble pattern has generally consisted of successive repetitions of four NRZ bits, [0011] or [1100]. This pattern is generally referred to as the “2T” pattern, a pattern that includes a magnetic transition every two bit periods.
Referring now to FIG. 1A, the typical hard disk drive sector generally includes four parts: Preamble 101, Sync 102, Encoded User Data 103, and Pad 104. The preamble is typically a single frequency pattern that may be used by the reading and/or decoding circuitry (e.g., automatic gain control [AGC] and/or phase-locked loop [PLL] circuitry) to establish initial gain and timing. The detector may also use the preamble to determine a correct initial state sequence.
The preamble is typically a 2T pattern, where “T” means one channel clock period. For example, a 1 Gbps channel rate translates into ins bit intervals (channel clock periods). In this case, a 2T pattern generally has a magnetic transition every 2 ns. Because transitions alternate positive and negative, one complete period of a 2T pattern spans 4T, or 4 ns in this example. The corresponding preamble frequency is 250 MHz. The typical preamble may span from about 48 to 120 channel bits.
Referring now to FIG. 1B, exemplary repetitive preambles are shown. Waveform 111 relates to a 2T repetitive NRZ pattern. Waveform 112 relates to a 3T repetitive NRZ pattern. Waveform 113 relates to a 4T repetitive NRZ pattern. Thus waveform 111 has a transition every two bit periods, waveform 112 has a transition every three bit periods, and waveform 113 has a transition every four bit periods. Therefore, waveform 111 has a period of 4T and a frequency of ¼T, waveform 112 has a period of 6T and a frequency of ⅙T, and waveform 113 has a period of 8T and a frequency of ⅛T.
The preamble may be followed by a sync mark, signaling the end of the preamble and the beginning of the encoded user data. The sync mark may typically span 18 to 27 channel bits. The user data may comprise a 512-Byte (4096-bit) block of binary data that has been scrambled and RLL (run-length limited) encoded. A pad of from about 32 to 64 channel bits may be appended at the end of the sector to keep the data transitioning (or “clocking”) through the channel and to resolve any ambiguous final code states.
Run-length limited encoding generally maps the user data into a new (and typically longer) sequence. This sequence generally follows a (d, k) run-length constraint (e.g., the encoded sequence has at least d 0s between 1 s and no more than k 0s between 1s). In conventional magnetic recording systems, (0, k) coded systems generally use a 2T preamble. In older systems, recording systems using (2, k) codes used a 3T preamble (i.e., successive repetitions of six NRZ bits, [000111] or [111000]). This was generally because the highest frequency allowed to be written by the (2, k) code was that of the 3T pattern.
As user bit density increases, the amplitude of the 2T preamble decreases with respect to the maximum amplitude in the data sector. In order to fit the full dynamic range of the signal into the quantizers without clipping, the synchronization system must therefore operate on a preamble pattern with lower amplitude and higher quantization noise. This results in a larger quantization error in the timing loop and/or gain loop in particular, and a larger synchronization error in general.
In the field of data storage media and mechanisms (e.g., hard disk drives [a type of read channel]), storage capacity must generally increase to remain competitive in the marketplace. As data storage capacity requirements increase, data bits (e.g., magnetically recorded bits) are generally packed more densely into the same recording medium dimensions to achieve higher capacity without increasing the size of the device. Therefore, it is desirable to improve the signal characteristics of the preamble pattern.
Another important parameter in the design of magnetic recording systems is the physical space between the recording head and the data recording medium (e.g., a rotating hard disk). The transducer in the head may perform both playback (read) and record (write) functions, and thus, may be known as a read/write head. The quality of recorded data bits (e.g., magnetic transitions) and the playback signal strongly depend on the clearance (or spacing) between the slider on the read/write head and the disk. This spacing is also known as flying height or the “fly height.” The fly height represents a critical design trade-off. If the heads are too high above the surface of the disk the heads then data errors may occur, but if the heads are too low, the risk of a head crash dramatically increases. The fly height of the heads must be maintained within tight parameters to ensure the reliability of the drive. Hard drives conventionally use Self-Monitoring, Analysis, and Reporting Technology (SMART) as a monitoring system to detect and report on various indicators of reliability, in order to anticipate failures. One such indicator is the fly height at which the heads float over the surface of the platters. Any trend downward in fly height may indicate the possibility of a head crash in the near future.
Various factors affect the read/write head-hard disk clearance during read and write operations, and can cause modulation of (or variances within) this spacing. In a magnetic data storage system, the speed of the disk rotation, the slider air bearing design, smoothness or roughness of the recording medium surface, operating altitude and temperature are some of the key factors.
FIG. 2 shows a conventional magnetic data recording and playback system 200, including read/write head 220 having write transducer (or coil) 230 and read transducer 240 electrically attached thereto. Electrical current passing through write coil 230 during a write operation generally heats the coil 230 and causes it to expand, reducing the spacing between write coil 230 and medium (or disk) 250. Protrusion of the recording element (write head) during the write process due to Joule heating and eddy-current losses may significantly reduce the flying height of sliders in hard disk drives. Such thermal expansion of the write coil 230 can also affect the position of the read transducer 240 relative to the disk 250. In some cases, the thermal expansion and contraction of the write coil 230 can be a primary factor in the variation of the fly height of the read/write head 220 and/or write coil 230.
After the recording system commands the drive servo to position the write head 220 on-track (e.g., at the beginning of a write operation) and a data read/write controller (such as a hard disk controller, or HDC) asserts a write enable signal (e.g., write gate or WG), a circuit such as a preamp sends current through the write head coil 230. The current passing through the coil 230 generates thermal power or energy, which causes the pole tip 235 to protrude towards the disk. The pole tip protrusion (PTP) generally reduces the magnetic spacing between the head and the disk 250.
On the other hand, when the write enable signal is deasserted, the current flow into or through) the write coil 230 is reduced or stopped, and the thermal energy stored in the pole tip 235 begins to dissipate into the air and the surrounding coil insulation material. The decay in thermal power (e.g., the rate of decrease in stored thermal energy in the coil 230) from the write operation causes the pole 235 tip to retract to its original position (e.g., to the original spacing). This modulation in spacing between the write head 220 or coil 230 and the data recording medium 250 can impact the data integrity and bit error rate (BER) of the drive.
One method to adjust the fly height spacing between a magnetic recording medium and the read/write head to maintain relatively constant spacing involves thermally heating the transducer region (in the write head) with a heater element. However, a need is felt for a method or technique that reliably measures fly height, while reducing or minimizing overhead in the system (e.g., additional or dedicated bits of code or circuitry, interrupts in data transmission/reception, etc.).
Needs therefore exist both to increase the accuracy and/or reliability of data reading systems (and in particular, of synchronization methods, firmware and/or hardware) to keep up with ever-increasing demands for increased data transmission speeds and data storage densities, and to measure the fly height of the read/write head(s) in such systems. It is desirable to satisfy these requirements with little or no additional disk area and/or circuitry in the read head.